There is a riveting scene in Steven Spielberg's 2012 film "Lincoln" in which the 16th president hotly demands that his aides do whatever it takes  deploy every ounce of leverage available  to obtain the last few votes needed to pass the Thirteenth Amendment.

"I am the President of the United States of America, clothed in immense power!" roars Abraham Lincoln, played by Daniel Day-Lewis. "You will procure me these votes."

The historicity of the quote is doubtful, but Lincoln's determination was not. The presidency does convey immense power, and Lincoln was relentless about deploying that power to achieve his great aim: the abolition of slavery in America.

What would Donald Trump do with such immense power?

Voters should think hard, of course, about the consequences of investing any candidate with the vast influence and clout of the presidency  an office much more formidable today than it was in 1865. Power tends to corrupt. That will be true whether the next president is liberal or conservative, male or female, Republican or Democrat.

But the authoritarian abuse of power in a Trump administration isn't just a theoretical possibility. Should the New York businessman win the presidency, it's a certainty. Trump's campaign, with its torrent of insults, threats of revenge, and undercurrent of political violence, is the first in American history to raise the prospect of a ruthless strongman in the White House, unencumbered by constitutional norms and democratic civilities.

When Trump's campaign manager, Corey Lewandowski, was arrested last week on misdemeanor charges of battery against reporter Michelle Fields, the candidate's reaction was typical. Though Fields's account was never in doubt  it was corroborated at once by an eyewitness (Washington Post reporter Ben Terris), by an audio recording, and then by security-camera video footage  Trump offered no apology and didn't rebuke his staffer. Instead he went on the attack: He claimed that Fields had "made the story up," he went out of his way to praise Lewandowski, and he gleefully trashed the journalists covering him as "disgusting" and "horrible people." Trump even hinted that he might sue Fields.

If this is how the Republican front-runner conducts himself when he is merely a candidate, what would he be like with the whole executive branch of the federal government at his command?

It is normal for passions to run high in election season. We're used to seeing candidates play to their base with animated rhetoric. What isn't normal is for a serious presidential contender, after being heckled by a protester, to tell a campaign rally: "I'd like to punch him in the face." What we're not used to seeing is a candidate who warns that if he fails to win the nomination at a contested convention, blood will flow: "I think you'd have riots," Trump said on CNN. "I think bad things would happen."

Barely-veiled blackmail is a Trump mainstay. The family of Chicago Cubs owner J. Joe Ricketts contributed to an anti-Trump PAC? "They better be careful, they have a lot to hide!" the candidate tweets. An independent group backing Cruz runs an ad featuring Melania Trump in an erotic pose? "Be careful, Lyin' Ted, or I will spill the beans on your wife!" another tweet warns.

Though Trump hasn't been nominated, let alone elected, he already signals that if he becomes president, anyone who opposes him will regret it. That includes House Speaker Paul Ryan, who had the temerity to criticize Trump's evasions about the Ku Klux Klan.

"Paul Ryan? I don't know him well," Trump remarked with a whiff of menace on Super Tuesday, "but I'm sure I'm going to get along great with him. And if I don't, he's going to have to pay a big price."

"You've got to give him credit. . . . He goes in, he takes over, he's the boss. It's incredible. He wiped out the uncle, he wiped out this one, that one. This guy doesn't play games"  Donald Trump's praise for North Korean dictator Kim Jong-Un

Trump's low-road brawling, thuggish tone, and gutter sexism are something new in American presidential politics. Dangerous demagogues are a species we have tended to associate with banana republics and military dictatorships. The fervent zealotry Trump's backers, the blind cult of personality that surrounds him, is shocking to many of us who always imagined that America was immune to the politics of caudillos and Dear Leaders. Now we know better. For a significant minority of American voters, an authoritarian brute who flirts with violence and has no scruples is just what they've been waiting for.

Imagine the presidency in the hands of such a man.

Trump has heaped praise on Vladimir Putin for being "a strong leader," looked back nostalgically at the bloody reign of Saddam Hussein, and insisted that it "would be so much better" if Moammar Gadhafi still ruled Libya. He has even applauded North Korea's sociopathic Kim Jong-Un for the "incredible" way he murdered his political rivals when he came to power.

Every president wants to get his way, and more than a few have bent some rules to the breaking point in the pursuit of their goals. But Trump holds out the prospect of a president for whom ends will always justify means, however dishonorable or scandalous or undemocratic. For many of his loyalists, nothing he does is beyond the pale; they are as blind to the grossness of his character as they are to the incoherence of his positions.

The rest of us should be thinking hard about what would happen if a man so unfit for leadership were to be clothed in the immense power of the presidency. And thinking even harder about how to prevent it.